Genes play a role in human disease.  Some diseases are caused by a mutation (an alteration) in the DNA sequence of a single gene.  In these cases a single protein is altered, and that is enough to cause disease.  Some diseases are caused by infection or damaging environments.  Many diseases have complex causes that involve multiple genetic mutations and/or environmental factors.  Muscular dystrophy is an example of a disease caused by a mutation in a single gene.  Asthma is an example of a disease with complex causation.